Marketers don't always play nice, especially when they're trying to take on one of their closest rivals.

Often the most brutal battles come in the form of comparative advertising -- the direct comparison of one company's product to another. Once the line is drawn, it's up to the opposition to either fire back or ignore its competitor.

Some companies have been duking it out for decades, like Coke and Pepsi. Others took decades to respond: Apple ran its first anti-PC advertisement in 1984, and Microsoft didn't fight back until recently -- it let hardware makers like Dell and HP do the fighting.

We've compiled some of the most intense marketing wars ever waged between two companies.